Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among secondary batteries, lithium secondary batteries with high energy density and voltage have been commercialized and widely used.
These lithium secondary batteries generally use metal oxides such as lithium cobalt-based oxides, lithium manganese-based oxides, lithium nickel-based oxides and the like as a positive electrode active material, and carbonaceous materials as a negative electrode active material, and such lithium secondary batteries are manufactured by disposing a polyolefin-based porous separator between a negative electrode and a positive electrode and impregnating the resultant structure with a non-aqueous electrolyte containing a lithium salt such as LiPF6 or the like. When the lithium secondary battery is charged, lithium ions of the positive electrode active material are deintercalated and are then intercalated into a carbon layer of the negative electrode. When the lithium secondary battery is discharged, the lithium ions of the carbon layer are deintercalated and are then intercalated into the positive electrode active material. In this regard, the non-aqueous electrolyte acts as a medium through which lithium ions migrate between the negative electrode and the positive electrode.
In particular, such an electrolyte basically requires stability within an operating voltage range of a battery, i.e., 0 to 4.2 V, and high ionic conductivity. The ionic conductivity of an electrolyte is an essential factor determining charge/discharge capacity of batteries, which depends on viscosity of the electrolyte and ion concentrations in the electrolyte. As viscosity of the electrolyte decreases, ions migrate more freely in the electrolyte and ionic conductivity thus increases.
Cyclic carbonate compounds such as ethylene carbonate have a high dielectric constant and play an essential role in realizing battery performance, for example, form an SEI layer during charge/discharge, but have a melting point equal to or higher than room temperature and thus have disadvantages of deteriorated ionic conductivity, high viscosity and poor wettability at low temperature. To solve these disadvantages, a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate or the like or a propionate compound, which has a low viscosity, is suitably used in combination.
However, through such solvent composition change alone, it is difficult to improve ionic conductivity of an electrolyte at a low temperature of −10° C., internal resistance increases and discharge characteristics are rapidly deteriorated upon high-rate discharge. Therefore, a variety of research is underway to improve low-temperature characteristics of batteries by suitably controlling the composition and mix ratio of a variety of solvents.